It is generally believed that the reaction of a zeolite with SiCl.sub.4 results in the removal of aluminum. It is suggested that not only Y-zeolite but also H-mordenite would undergo the above-mentioned reaction in accordance with the following: EQU Na.sub.x Al.sub.x Si.sub.y O.sub.x+y +SiCl.sub.4 .fwdarw.Na.sub.x-1 Al.sub.x-1 Si.sub.y+1 O.sub.x+y +AlCl.sub.3 +NaCl
[see "Zeolite no Kagaku to Oyo", H. Tominaga, ed. pp. 100-102, Mar. 30, 1987 (Kodansha)].
On the other hand, it is known that the activity of a zeolite to be used as a catalyst largely depends on the amount of Al present in the crystal skeleton thereof and that the activity decreases as the amount of Al present decreases accompanied by a decrease in the number of acid sites. In this case, however, the acid strength increases. It is further known, on the other hand, that a zeolite having a pore size suitable for use as a catalyst should be selected.
In order to control the pore size of a zeolite, introduction of a cation or reaction of the hydroxyl groups of the zeolite with, for example, tetramethylsilane or tetramethoxysilane have been proposed. However, the former method results in a decrease in the amount of the solid acid, while the reactants required in the latter method are expensive organic compounds (see "Zeolite no Kagaku to Oyo", supra, p. 105).
Therefore, it has been considered that the decrease in solid acid sites caused by the introduction of a cation for the controlling of the pore size could be recovered by effecting another treatment (see "Zeolite no Kagaku to Oyo",supra, p. 105).